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LX90 Dec lag when changing directions.

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#1 gcs111

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Posted 24 May 2023 - 10:10 PM

Hi....

I recently picked up an older LX90 and noticed when I change Altitude (Declination) direction with hand Ctrl.  there's a lag in movement. 

For example if I set the speed to #4, it takes 5-6 seconds before I see any movement at the eyepiece.  I looked at the worm gear when changing directions with a faster speed.   I can see the worm gear move away from the spur gear when changing the direction, then moves back into the spur gear to make good contact.  After the initial change all movements in the new direction are OK.  If I again change the direction the other way it happens all over again.

Any ideas what's going on?

 

Thanks for any replies,  Gerry


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#2 Skywatchr

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Posted 25 May 2023 - 06:56 AM

Hi....

I recently picked up an older LX90 and noticed when I change Altitude (Declination) direction with hand Ctrl.  there's a lag in movement. 

For example if I set the speed to #4, it takes 5-6 seconds before I see any movement at the eyepiece.  I looked at the worm gear when changing directions with a faster speed.   I can see the worm gear move away from the spur gear when changing the direction, then moves back into the spur gear to make good contact.  After the initial change all movements in the new direction are OK.  If I again change the direction the other way it happens all over again.

Any ideas what's going on?

 

Thanks for any replies,  Gerry

This is a mechanical issue with the way the worm housing is mounted at the pivot screw.  What you describe as the "spur" gear is the "wheel" gear.  That movement can be caused by two things.

1. The backstop is allowing the worm to "climb" away from the wheel too far.

2. The pivot screw has too much play where the worm block itself is too loose with respect to tilt. It will actually "twist" a little instead of remaining flat which also allows the worm to "climb" the wheel, but in a different direction. Both will in their own way add to the extra backlash.

 

Now, you can also use what is called "backlash compensation" in the Autostar to help with that.  That is explained in the link below...

 

http://www.weasner.c...g-percents.html

 

Also be sure the Autostar firmware is updated to at least 2.1EK or newer to have backlash compensation. Preferably use StarPatch software with the 43G firmware, and the patch for the firmware.  It is free and no need to "register" the StarPatch software.  It is much better and safer than the Meade ASU software.

It has been a long time since I worked on an LX90, but resolving at least one of the two mechanical issues above will greatly improve the backlash-like behavior.  The backlash compensation in the handbox merely "speeds up" the motor momentarily to take up the slack.  It's a trial and error procedure because of the varying amounts of backlash for any individual scope.

 

Also the grease may be getting dry and worn off the surface of the gears causing a little more friction than there should be. Which of course would produce more "climb" than normal.

 

I myself would fix all mechanical issues first, then employ the backlash compensation to eliminate any remaining backlash.  If you aren't able to do the mechanical part, just use the backlash compensation feature. waytogo.gif



#3 gcs111

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Posted 26 May 2023 - 02:08 AM

Thanks Skywatchr.....I recently cleaned and re-lubed the worm and spur (wheel gear). Reoiled the pivot screw and block screw. No play in pivot screw side and the block screw side moves freely.
So I think this isn't the problem.
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#4 gcs111

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Posted 26 May 2023 - 02:12 AM

What is the "backstop" that you mention?



#5 gcs111

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Posted 26 May 2023 - 02:13 AM

Is the spring that pushes up on the worm gear loosing it's springiness?


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#6 Skywatchr

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Posted 26 May 2023 - 06:17 AM

Is the spring that pushes up on the worm gear loosing it's springiness?

It could be.  I looked at some photos of the LX90 and it appears there isn't a backstop screw.  Just the spring that is supposed to keep the worm in contact with the wheel, but yet allow movement in the case of overload or something else.

The LX200 Classic has backstop screws to limit the amount of worm climb.    So anyways, use the backlash compensation in the handbox to try and eliminate it's effects.



#7 Skywatchr

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Posted 26 May 2023 - 06:24 AM

Thanks Skywatchr.....I recently cleaned and re-lubed the worm and spur (wheel gear). Reoiled the pivot screw and block screw. No play in pivot screw side and the block screw side moves freely.
So I think this isn't the problem.

There is a point where the worm block can move "too" freely and actually rock a little.  It doesn't take much lateral movement at all to add to the backlash issue. When I had an LX90 years ago, I had to actually shim the worm screws to stop the lateral movement.  That was in the RA drive.

At least in the DEC, you can weight the scope a little nose heavy to keep constant weight on the DEC worm/wheel.  But that only works until you get close to vertical.




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